1. Field of the Invention
This invention relates to an insulated wire used in the manufacture of a coil, such as a deflection yoke for a television, or computer display unit, and more particularly, to a self-bonding insulated wire having a self-bonding layer as its outermost layer.
2. Description of the Related Art
A self-bonding insulated wire has a bonding outermost layer formed on a conductor, or on an insulating layer on the conductor. A self-supporting coil can easily be formed from it if, after coiling, its bonding outermost layer is melted or swollen by e.g. electrical heating, hot air blowing, or solvent treatment to fuse every two adjoining turns of the wire together. Therefore, this kind of insulated wire does not require any step of impregnation with an insulating varnish, or the like, but makes it possible to realize an improved productivity in the manufacture of a coil for an electric apparatus and a reduction of its cost, and has come to be widely used as a magnet wire for a domestic electric appliance, OA apparatus, or other electric equipment. The self-bonding insulated wire typically comprises a conductor, an insulating layer formed thereon by repeating a plurality of times the application and baking of an insulating varnish, such as a polyurethane, or polyesterimide varnish, and a bonding outermost layer formed thereon by applying and baking a bonding varnish. It has been usual to employ, for example, an epoxy resin varnish, or a copolyamide resin varnish as the bonding varnish. A self-bonding insulated wire made by using a polyamide resin varnish as the bonding varnish is particularly widely used owing to a good bonding strength exhibited by a coil formed from it by winding, bonding and press molding.
There is a demand for a self-bonding insulated wire having a high resistance to heat deformation even at a high temperature to make a small, heat-resistant high-voltage and high-frequency coil having an improved space factor for an electric apparatus. A deflection yoke for a television, or computer display unit, among others, requires as a high-definition and high-frequency part thereof a coil which can resist heat (at a temperature of at least 105.degree. C. at present, but at least 120.degree. C. in the future) without making any change in dimensions, or a coil which hardly makes any change in dimensions as formed, but is close in dimensions to a winding die. Thus, there is a strong demand for a self-bonding insulated wire which has high levels of heat deformation resistance and bonding strength at a high temperature, and can form a coil which is substantially free from any deformation.
A conventional self-bonding insulated wire having a bonding layer formed from an epoxy resin varnish can make a coil which is hardly deformed, but is close in dimensions to a winding die. This coil has, however, an undesirably low level of bonding strength and a heat deformation temperature which is as low as 90.degree. C.
A conventional self-bonding insulated wire having a bonding layer formed from a copolyamide resin varnish can make a coil having a good bonding strength and a heat deformation temperature which is as high as 120.degree. C. Upon cooling to room temperature, however, the coil is deformed and becomes larger than the size defined by the winding die, and its deformation causes it to be distorted. If such a coil is used as a part of a deflection yoke in a high-definition display unit, or the like, it is likely to return from its distorted form and thereby cause misconvergence to occur on the screen.
A conventional self-bonding insulated wire made by applying and baking a polyamide resin varnish on an insulated wire calls for a considerably strictly controlled baking job, since its properties are greatly dependent on the baking conditions, and the resin forming its bonding layer is easily decomposed if its baking temperature is higher than usual. A coil formed from a self-bonding insulated wire having a partly decomposed resin layer is heavily deformed by springback immediately after its formation, as compared with a normal one. A deformed deflection coil is difficult to incorporate in e.g. a television, or computer display unit. Moreover, the deformation of the coil by the heat resulting from the use of e.g. a display unit causes misconvergence to occur on the screen.
The use of a self-bonding resin having a higher level of heat resistance (or a higher softening temperature) has been proposed to overcome the problems as pointed out above, but the higher heat resistance requires a higher bonding temperature for satisfactory bonding, and thereby brings about an increase of energy consumption and the heat deterioration of the insulating layer. Japanese Patent Application Laid-Open No. Hei 7-182929, for example, discloses a bonding varnish comprising 100 parts by weight of a copolyamide resin, and 5 to 50 parts by weight of nylon 12 having a specific relative viscosity. The presence of at least 5 parts by weight of nylon 12, however, has an adverse effect on the copolyamide resin, and is particularly undesirable as it brings about a lower bonding strength. It has also been found that a coil formed from a self-bonding insulated wire having a bonding layer composed of the proposed varnish is inferior in dimensional accuracy (or closeness in dimensions to the winding die used) and distorting resistance to a coil of a wire having a bonding layer of an epoxy resin.